Using string instead of char array with read() and write() in a struct to file

Question

Is there a way a string could be used instead of a char[] array in the below struct, and still be saved to a file using read and write functions without getting a runtime error?

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

struct details
{
    char name[30];
    int add_year;
};

const char * file = "students.dat";

int main()
{
    details x;
    ifstream ifile;
    ifile.open(file,ios_base::in|ios_base::binary);
    if(ifile.is_open())
    {
        cout<<"Contents of "<<file<<" is"<<endl;
        while(ifile.read((char *)&x,sizeof x))
        {
            cout<<x.name<<" - "<<x.add_year<<endl;
        }
        ifile.close();
    }
    ofstream ofile(file,ios_base::out|ios_base::app|ios_base::binary);
    if(!ofile.is_open())
    {
        cerr<<"Can't open "<<file<<endl;
        exit(EXIT_FAILURE);
    }
    cout<<"Name"<<endl;
    cin.get(x.name,30);
    cout<<"Year added"<<endl;
    cin>>x.add_year;
    ofile.write((char *) &x, sizeof x);
    ofile.close();
    cin.get();
    return 0;
}

Look up serialization. That will show you how this should be done — NathanOliver
– NathanOliver, Commented Dec 7, 2016 at 19:22
Most of it where only about the definition "converting objects into binary data streams...". — Kinyo356
– Kinyo356, Commented Dec 7, 2016 at 19:31
A character array can have advantage because it is fixed in length. — Thomas Matthews
– Thomas Matthews, Commented Dec 7, 2016 at 19:32

Remy Lebeau · Accepted Answer · 2016-12-07 21:11:33Z

1

A std::string (or any other dynamic sized container) contains a pointer to its character data that is stored elsewhere in memory. If you were to read()/write() the std::string itself as-is, you would only be reading/writing the pointer value, not the actual character data. This is not the case with a fixed array.

In order to use std::string in your struct, you would have to (de)serialize the content of the struct back and forth so you can take dynamic data into account.

For example:

#include <iostream>
#include <fstream>
#include <string>
#include <cstdint> 

using namespace std;

struct details
{
    string name;
    int32_t add_year;
};

istream& operator>>(istream &in, details &out)
{
    int32_t len;
    if (in.read((char*) &len, sizeof(len)))
    {
        out.name.resize(len);
        if (len > 0) in.read(&(out.name[0]), len);
        in.read((char*) &(out.add_year), sizeof(out.add_year));
    }
    return in;
}

ostream& operator<<(ostream &out, const details &in)
{
    int32_t len = in.name.size();
    out.write((char*) &len, sizeof(len));
    out.write(in.name.c_str(), len);
    out.write((char*) &(in.add_year), sizeof(in.add_year));
    return out;
}

const char * file = "students.dat";

int main()
{
    details x;

    ifstream ifile;
    ifile.open(file, ios_base::binary);
    if (ifile.is_open())
    {
        cout << "Contents of " << file << " is" << endl;
        while (ifile >> x)
        {
            cout << x.name << " - " << x.add_year << endl;
        }
        ifile.close();
    }

    cout << "Name:" << endl;
    getline(cin, x.name);
    cout << "Year added:" << endl;
    cin >> x.add_year;

    ofstream ofile(file, ios_base::app|ios_base::binary);
    if (!ofile.is_open())
    {
        cerr << "Can't open " << file << endl;
        exit(EXIT_FAILURE);
    }
    ofile << x;
    ofile.close();

    cin.get();
    return 0;
}

answered Dec 7, 2016 at 21:11

Remy Lebeau

610k36 gold badges516 silver badges875 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Kinyo356 Over a year ago

Thanks. It works. But does that mean for every string type being a member of a struct, I must make a separate definition of using read and write like you did with the len in both functions?

Remy Lebeau Over a year ago

@Kinyo356: Reading/writing the string length is just a convenience, not a requirement. You could just read/write a null terminator instead, for instance. Using a length makes memory management a little easier when reading back. But it depends on your particular storage format. But yes, in general, you must serialize any kind of dynamically allocated data, which includes std::string, variable length arrays, etc.

Kinyo356 Over a year ago

OK. I appreciate the explanation

Collectives™ on Stack Overflow

Using string instead of char array with read() and write() in a struct to file

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related